The present invention relates to a rotary feeder system and in particular to a pressurized pneumatic conveying system capable of conveying a constant quality of powdered or granular materials continuously at low velocity and in dense phase using a high-pressure air source. More specifically, it relates to a pressurized pneumatic conveying system in which a conventional rotary feeder is used. A center axis of the rotor is made eccentric with respect to a rotor driving shaft which is pivotally supported in a cylindrical casing. The casing has upper and lower openings that permit the blade tips of the rotor to contact the inner circumferences of the casing adjacent the upper and lower openings by means of the differential in pressure that is established between the upper and lower passages. A compressor is used as the air source in the system. By reducing the friction between the pipe wall and the material being conveyed, a constant flow of material through the system can be realized. The material may be fines, composites or the like.
In general, low-velocity transfer in a pressurized pneumatic air conveying system for handling powdered or granular materials is defined as a system in which the conveyed quantity is small, the transfer generally takes place in the range of about 2 to 12 kg/m.sup.2 of a mass flow rate of conveying air. The flow rate becomes smaller as the diameter of conveying pipe decreases. High velocity transfer is defined as a pneumatic conveying system which is a dilute phase conveying system mainly suitable for a medium distance transfer. This type of system usually employs a Roots blower or turbo-blower having a discharge air pressure of 1 kg/cm.sup.2 or less as an air source and the transfer takes place at the mass flow rate of 12 kg/cm.sup.2 or more.
In the aforesaid low-velocity conveying system, a compressor of about 5 to 7 kg/cm.sup.2 normal pressure is widely used. Cellar-type or fluxo-type blow tanks, which are subject to pressure vessel regulations, are also needed to meet the requirements covering equipment for feeding and storing various granular materials.
Referring to the conventional low-velocity conveying system of FIG. 9, there is shown a configuration in which a twin-body cellar-type tank is employed. A powdered or granular material to be conveyed is charged into a feed hopper disposed at the top of the system. The material is introduced into the cellar-type blow tank (20b) under atmospheric pressure by opening and closing of a cone valve.
At this time, from a multi-phase feed pipe (22), connected to another blow tank (20a), the material is sent out in a lump or plug (hereinafter merely referred to as a plug) and conveyed slidably in a conveying line system (23) connected to the end of said feed pipe (22).
During this conveying process, a primary air nozzle (21) is connected to compressed air piping (26) which is in communication with the high-pressure air source together with a secondary air nozzle (24) and back pressure nozzle (25). The transfer is effected in a plug-flow fashion as the plugs move slidably in the pipe. At least three or more spaced nozzles, such as the primary, secondary and back pressure nozzles must be adjusted, one relative to the other, to effect the batch transfer.
However, feed volumes to the conveying line fluctuate, thereby causing plugs in the conveying line to break, and there is little that can be done to prevent this from happening.
In order to solve such problems in a batch switching system, it has been proposed to construct the blow tanks in two vertical stages which are connected in series to effect continuous feeding by switching upper and lower dampers.
In this construction, however, in addition to the unequal plug lengths and plug intervals, variations of the feed volume by forced feeding depend on the adjustment of compressed air flow so that it has a mass below one-thousandth of the feed volume. Since the feeding volume is not fixed mechanically, the fluctuations in the feeding volume are difficult to prevent.
Moreover, since the air-tight blow tanks are stacked in two stages and installed vertically as pressure vessels, the cost for filling the materials to be conveyed to the tank-top hopper is very expensive when compared to the twin-body type blow tanks.
Next, referring to a well-known configuration of a high-velocity conveying system shown in FIG. 10, powdered or granular materials are charged into a hopper (27) under atmospheric pressure and are discharged to a lower acceleration mixing chamber (29) from an outlet port at its lower end by rotation of a low-pressure rotary feeder (28). The material is then conveyed in a fixed amount to a cyclone separator and a storing silo continuously through a conveying line (23).
In this case, a turbo-blower or roots blower having a capacity of about 0.3 to 1.0 kg/cm.sup.2 is generally used.
Atmospheric pressure is found on both the charging side and discharging side of such conventional type low-pressure rotary feeder (28) and the differential pressure depends largely on characteristic differences of powdered or granular materials to be conveyed. It is usually below 0.7 atm and the air source pressure can be supplied at 0.5 to 1.5 kg/cm.sup.2 as the limit, thereby the controlled mass transfer can be secured.
The present rotary feeder has a simple construction in which a rotor is supported in a casing, and a constant supply of granular material is handled in an air-tight manner by simply intercepting upper and lower passages having the required pressure differential outside the casing. Since any fixed amount of material can be supplied by adjusting the speed of the rotor, the system is widely used for supplying powdered or granular materials to the conveying system and for mixing the same therein.
Generally, a small clearance between the rotor and casing is preferred since the rotary feeder is designed to intercept pressure of the conveying system and drop the powdered or granular materials by gravity. A suitable clearance is required therebetween for solving difficulties in manufacturing and assembling the casing and rotor, or operational difficulties caused by excessive rotational resistance attributable to intrusions of powdered or granular materials, thermal expansion of the rotor and so on.
Leakage of pressurized gas increases as the pressure differential between upper and lower portions of the casing become larger. The apparent specific gravity of the materials to be handled is reduced as the drop of materials to be handled decreases or diverges due to blowback of the leaked gas, resulting in inconstant feeding.
The limit of the pressure differential between the upper and lower passages outside the casing is usually below 0.7 atm. For example, in the case of materials to be handled whose apparent specific gravity is about 0.5 T/m.sup.3, the practical limit is about 6000 to 7000 mm H.sub.2 O, and about 3000 to 4000 mm H.sub.2 O and is thus unsuitable for use in a long-distance high-pressure conveying system.
In order to reduce the clearance between the casing and rotor, it is proposed to coat the rotor tips with fluororesin to reduce rotational resistance, or to embed the tips with carbon chips and protrude them outward by means of springs.
However, in the aforesaid construction, problems are encountered in abrasive resistance and reliability, thus the rotary feeder is still deemed to be unsuitable for use in long-distance high-pressure conveying system.
While recently in the pneumatic transfer of finely powdered or granular materials such as composite materials whose surfaces are activated, higher conveying pressures are demanded to attain higher pneumatic conveying efficiency.